What do you do as a parent when you catch another child hitting your child? My son is two and the last two?

times I went to Macdonald play place I watched a older boy hit and push my son down. Inside I was so angry but I didn't do much. I just walked over very close so the other child could see that I was watching him. Today when I saw another little boy bash my son in the head with a toy I was mad. I told the boy no hitting. I just wanted to know what other parents would do.